This application claims the priority of European patent document 06 005 846.8, fled Mar. 22, 2006, the disclosure of which is expressly incorporated by reference herein.
The invention relates to satellite systems that provide navigation services. Examples of such systems are generic Global Navigation Satellite Systems (GNSS) such as Medium Earth Orbit (MEO) constellation based systems (like the American GPS, the Russian GLONASS or the European Galileo), and generic Satellite Based Augmentation Systems (SBAS) like for instance the European EGNOS, the American WAAS or the Japanese MSAS. However, these are only examples. The invention is not restricted to these systems. The invention can apply in all areas where navigation services are deemed to meet performance requirements, including accuracy, integrity and continuity criteria or any combined subset of these criteria. It can apply in many GNSS application domains, including but not limited to aviation, telecommunications, maritime operations, power grid control, terrestrial fleet management, precision agriculture, defense and homeland security, intelligent transport systems and other GNSS applications known in the art.
For navigation services in aviation and other safety of life (SoL) domains, the service performances are defined in terms of user position accuracy (U_Acc), user position continuity (U_Cont) and user position integrity (U_Int) versus predefined alert limits (U_AL).
According to the applicable aviation and other SoL standard requirements, the following definitions apply:
User position error (UE) refers to the difference between a measured or estimated position and the true position at any user location. User position accuracy (U_Acc) refers to 0.95 percentile (“2-sigma”) of user position error (UE) distribution model at a given user location.
User position continuity risk (U_Cont) refers to the probability of having, in a coming bounded time interval, a discontinuity of the positioning service (e.g., a probability of 10−5 within a time interval of 15 seconds).
Signal in space continuity risk (SiS_Cont) refers to the probability of having, in a coming bounded time interval, a discontinuity of the signal in space of a given Satellite (e.g., a probably of 3-10−6 within a time interval of 15 seconds). The discontinuity can be caused by various factors, including signal broadcast interruption or integrity flag being raised by the ground against the considered satellite.
User position integrity risk (U_Int) refers to the probability of having, in a coming bounded time interval, a position error equal or beyond defined vertical/alert limits (U_AL) (e.g., a probability of 10−7 within a time interval of 150 seconds).
Signal in space integrity risk (SiS_Int) refers to the probability of having, at user level, in a coming bounded time interval and for a given satellite, a signal in space anomalous error (e.g., a probability of 10−6 within a time interval of 150 seconds). The signal in space integrity risk (SiS_Int) can materialize through various factors including on board clock instabilities, on board transponder path delay instabilities, unpredicted maneuver, ephemeris and clock errors, etc. The distribution of magnitude of unflagged signal in space feared event is over-bounded thanks to (SiS_Int) and to the defined signal in space Alert Limit (SiS_AL).
Signal in space static integrity risk (SiS_IntS) refers to the probability of having, at user level, in a coming bounded time interval and for a given satellite, a signal in space anomalous error, with dynamic lower than SiS_VRL, and no ground segment flag provision under the time to alert requirement conditions (e.g., a probability of 10−6 within a time interval of 150 seconds). The signal in space static integrity risk (SiS_IntS) can materialize through various factors including on board clock instabilities, on board transponder path delay instabilities, unpredicted maneuver, ephemeris and clock errors, etc. The distribution of magnitude of unflagged signal in space static feared event is over-bounded thanks to SiS_IntS and to the defined signal in space Alert Limit (SiS_AL).
Signal in space dynamic integrity risk (SiS_IntD) refers to the probability of having, at user level, in a coming bounded time interval and for a given satellite, a signal in space anomalous error, with dynamic higher than SiS_VRL and no ground segment flag provision under the time to alert requirement conditions, e.g., a probability of 10−6 within a time interval of 150 seconds. The signal in space dynamic integrity risk (SiS_IntD) can materialize through various factors including on board clock instabilities, on board transponder path delay instabilities, unpredicted maneuver, ephemeris and clock errors etc. The magnitude of the signal in space feared event is over-bounded thanks to SiS_IntD, to the defined time to alert TTA and to the defined signal in space variation rate limit SiS_VRL.
Especially, the navigation performance requirement for SoL services includes user position accuracy (U_Acc), user position continuity (U_Cont) and user position integrity (U_Int) versus predefined vertical and horizontal alert limits (U_AL). At any given point in time and space, the navigation SoL service is declared available only if the user position accuracy (U_Acc), the user position continuity (U_Cont), and the user position integrity (U_Int) are below thresholds defined by the applicable SoL standard.
Signal in space accuracy (SiS_Acc) refers to a quantity characterizing the spread of the signal in space error distribution in a fault free mode.
Signal in space monitoring accuracy (SiS_MA) refers to a quantity characterizing the spread of the signal in space monitoring error distribution. Combined with defined SiS_AL and SiS_Int/SiS_IntS, these quantities provide an over-bound of the signal in space error in a failure mode.
Signal in space alert limit (SiS_AL) refers to the estimated signal in space error limit above which the ground component of the satellite system provides an integrity flag (IF). Combined with defined SiS_MA and SiS_Int/SiS, IntS, these quantities provide an over-bound of the signal in space error in the failure mode/static failure mode.
Signal in space variation rate limit (SiS_VRL) refers to the estimated signal in space error variation rate limit above which the ground component of the satellite system provides an integrity flag (IF). Combined with defined SiS_IntD, SiS_MA and TTA, these quantities provide an over bound of the signal in space error in a dynamic failure mode.
In the dynamic failure mode, the product SiS_VRL*TTA is the equivalent of defined quantity SiS_AL in the static failure mode.
Time to alert (TTA) refers to the maximum time delay between the appearance of an alert condition and the reception at user level of the subsequent integrity flag provided by the ground component of the satellite system.
Estimated signal in space error (eSISE) refers to the ground component estimation of the signal in space error SISE.
Finally, estimated signal in space variation rate refers to the ground component estimation of the signal in space variation rate (eSISVR).
The prior art has the disadvantage that first it makes no distinction between static and dynamic failure modes (privation of easy detection of a particular kind of failure (ramps, steps)), and second it supposes all of the defined above quantities are hard-coded in user algorithms, except signal in space accuracy (SiS_Acc) and signal in space monitoring accuracy (SiS_M). Hard-coded values have the disadvantages that the system is not adaptive to future evolution, that it is not adaptive to receiver autonomous integrity monitoring facility, that it is not adaptive to low satellites underperformances, and that large margins are necessary, inducing oversizing.
One object of the invention is to provide a satellite system and a process capable of determining the integrity risk and the continuity risk at any user location.
This and other objects and advantages are achieved by the method and apparatus according to the present invention, which provides for the broadcast of all information necessary for risk calculation, enabling the system to be adaptive to future satellites performances evolution. (No value should be hardcoded.)
It is a further advantage of the present invention that the broadcast of different information relevant to different classes of receiver (variable RAIM capabilities, i.e., capabilities referring to ramp detection, receiver coupled to inertial system, etc.) enables the service to be adaptive to user specificity.
It is a further advantage of the present invention that the broadcast of all information necessary to risk calculation makes it possible to reduce the margin which should have been taken with hard coded, and thus to optimize the system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.